1. Field of the Invention
The present invention relates to an ultrasound diagnostic apparatus which can obtain an ultrasound observation image of a body tissue through a mechanical scanning by an ultrasound probe inserted in the body cavity of a human being.
2. Description of the Related Art
In this type of ultrasound diagnostic apparatus, two ultrasound beam scanning systems are known: an electronic and a mechanical system. As known in Published Unexamined Japanese Patent Application No. 57-168648, the apparatus using a mechanical system is of such a type that a probe for transmitting and receiving an ultrasound is rotatably provided in a chamber defined by an insertion section tip cover of an instrument, and connected to a flexible drive shaft passing through the insertion section's channel. A rotational force of a motor on an operation side of the instrument is transmitted via the drive shaft to the ultrasonic probe. By so doing, the ultrasound probe is rotated to change the direction of its transmitting ultrasound beam and that of its receiving echo in its scanning motion.
An ultrasound transmitting medium is filled in the chamber set out above. The interior of the chamber communicates with the channel through which the drive shaft passes with the ultrasound transmitting medium filled in a resultant communication space provided by the two.
The drive shaft is formed of a tightly coiled metal wire and has its own flexibility. When, therefore, the ultrasound transmitting medium is filled in the communication space, tiny air bubbles are liable to stay trapped between the wire turned portions and in the coil's interior. The resultant structure, if being assembled in such a situation, allows the air bubbles to gradually enter the chamber where the transmitting/ receiving probe is located. With the air bubbles so intruded in the chamber, the ultrasonic beam transmitted and received by the probe is scattered or damped by the air bubbles, failing to obtain a better transmission and reception of the ultrasonic wave.
Further, dust is also produced due to, for example, the frictional contact of the rotating drive shaft in use with a channel member and enters the chamber via the channel. The dust causes the scattering or damping of the transmitting and receiving ultrasonic beam, failing to obtain a better ultrasonic image.
In order to solve this problem, a proposal is made to provide an air bubble trap between a chamber and a channel as disclosed in U.S. Pat. No. 4,807,634 (corresponding to Published Unexamined Japanese Patent Application No. 62-201145).
The provision of the air bubble means cannot completely avoid the entry of the air bubbles and dust.
In order to overcome this inconvenience, Published Unexamined Japanese Patent Application No. 58-152547 proposes to provide a sealing member between a chamber having an ultrasonic probe therein and a channel through which a drive shaft passes whereby air bubbles are prevented from entering the chamber.
The use of the sealing member has a drawback in that it is not possible to fill a ultrasound transmitting medium from the chamber side while preventing the ingress of air bubbles into the chamber, by the use of suction from a guide passage, so that they do not remain in the chamber. Further, the sealing member is provided immediately behind the chamber and it is difficult to fill the ultrasound transmitting medium into the guide passage from behind the sealing member. Since the guide passage normally provides a space where air is occupied, there is a risk that the drive shaft will not be smoothly rotated in the guide passage due to a lack of adequate lubrication relative to the drive shaft.
In use an instrument's insertion section is bent and, therefore, the drive shaft is rotated in a manner to be intensely in frictional contact with the inner surface of a tube for guide passage. A pliable material, such as polyurethane or teflon, of which the guide tube is made, wears out due to a lack of adequate lubrication on the drive shaft, causing many tiny particles originating from the wear to float in the passage.
The sealing member shuts off a communication between the chamber and the guide passage and, as a whole air layer occupies a location behind the sealing member, "breathing" occurs between the interior of the chamber and the guide passage eve if a slight pressure differential is developed across the chamber interior and the passage. This causes a gradual ingress into the chamber of the air bubbles and particles originating from the wear, disturbing an ultrasonic image and hence offering a bar to diagnosis.